When extracting oil and gas from underground formations, boreholes are drilled through the underground formations until the oil or gas bearing strata are reached. The techniques for drilling such boreholes are well-known and typically involve the use of a drilling fluid both to assist in the drilling action and removal of drilled cuttings, and in the stabilization of the borehole by hydrostatic pressure and formation of a mud cake around the surface of the borehole in porous formations.
There are a number of issues relating to the wall quality of the borehole. Equipment sticking is one major issue that occurs regularly in various wells. This problem may occur with drilling equipment, wireline tools and cable, casing and completion equipment, etc. Various causes of sticking exist, the most common being “differential sticking”. In this situation, sticking is due to a combination of differential pressure between the well-bore and formation and poor quality of the mud cake. Differential sticking occurs often when the mud cake is fragile, thick, and unconsolidated.
With wireline equipment, differential sticking may also occur on the cable itself when the equipment is in static condition (for example during the pressure testing of the formation). Sticking is acerbated by the fact that the cable can cut through the mud cake during running in and out of the well.
Other reasons for equipment sticking (especially while tripping-out of the well) is geometrical irregularity of the well such as ledges, key seats, etc. It is not always easy to detect the presence of these irregularities and to smooth them out by “machining” effect of the drill-bit when drilling the borehole. Thus they can remain even after the well has been drilled.
Ensuring proper sealing between the well-bore and the formation is critical not only to reduce the risk of sticking but also for limiting the mud loss. With high porosity and permeable formations at low pressure, mud loss control may depend also of the quality of the interface and mud cake. In some situations, these qualities are not achieved and mud loss may be difficult to control. Mud loss in highly fractured rock (of low pressure) may be also difficult to control with either loss circulation material mixed within the drilling mud, or by special fluids (or pastes) such as cement and polymer injected at the best estimated position.
In other cases, the quality of the cement job used to line the borehole may not be optimum to ensure proper zone isolation. A number of reasons for this may exist. Well-bore irregularities such as wash-outs may make mud removal prior to cement placement difficult. Thick and soft mud cake may also be part of the problem for lack of zone isolation as it can create “mud channels” left parallel to the well axis. Improper formation sealing by mud cake also increases the risk of gas channeling during cement setting. These channels may create leakage paths between zones. Even with current technology, problems with zone isolation can be difficult to detect and repair.
Current drilling practices may also contribute to well-bore irregularity may. For example:                High flow rates may generate wash-outs in weak formations.        Hole enlargement may be provoked by aggressive drilling (drill-string resonance, BHA whirling, and the like).        A succession of dog-legs (with occasional ledges) can be created when drilling with steerable motors.        Rough surfaces can be obtained when drilling with steerable motors in rotary mode.        
These hole conditions affect friction between devices (drill-sting, casing, etc.) in the hole and the bore-hole. In some case, drilling string or casing string cannot move downwards to too high friction, Wireline logging may also be of low quality due to these wall problems.
There have been a number of proposals for dealing with borehole wall quality during drilling. Examples can be found in WO 2004/057151, U.S. Pat. No. 2,776,111 and SU 1 361 304.